The present invention relates to a copier.
FIG. 6 is an overall arrangement view of a conventional copying machine. A platen 1 made of a pane of transparent glass is provided in the upper center of the copying machine. A scale plate 2 to designate a setting position of document D is provided at the left end of the platen 1. A document cover 3 to cover document D placed on the platen 1 is provided in the upper portion of the copying machine. In this case, the document cover 3 can be opened and closed with respect to the platen 1.
Document D is placed on the platen 1 in accordance with the designation on the scale plate 2, and covered with the document cover 3 so that the movement of document D is restricted. A display unit 4 (not shown) in which liquid crystals of the reflection type are used is provided close to the center on the viewer's side of the platen 1.
In the copying machine, under the platen 1, there is provided a first mirror unit 12 having an exposure lamp 10 and a first mirror 11 in such a manner that the first mirror unit 12 can be linearly moved to the right and left in parallel with the platen 1 so as to scan the overall surface of document D. A second mirror unit 15 is integrally composed of a second mirror 13 and a third mirror 14. In order to maintain a predetermined optical path length, the second mirror unit 15 is moved to the right and left at half speed of the first mirror unit 12 in parallel with the platen 1.
Reflected light sent from document D placed on the platen 1 is reflected by the first, second and third mirrors 11, 13, 14. Then, the reflected light is incident upon a primary lens 16. After that, the reflected light passes through a fourth mirror 17, and is incident upon a photoreceptor drum 20 through a slit 18.
A charging electrode 21 uniformly charges the photoreceptor drum 20. Accordingly, electrostatic latent images are successively formed on the photoreceptor drum 20 rotated clockwise when exposure light is sent from the aforementioned optical system. A developing unit 22 develops the electrostatic latent image on the photoreceptor drum 20 to a visual toner image.
A sheet feeding device to feed transfer papers includes: sheet feeding cassettes 23 (in this case, two sheet feeding cassettes are shown in the drawing); first sheet feeding rollers 24 to feed transfer papers P one by one from the sheet feeding cassettes 23; second sheet feeding rollers 25 to feed transfer sheet P to the photoreceptor drum 20; and a guide plate 26 provided between the sheet feeding cassettes 23 and the second sheet feeding rollers 25, and a guide plate 27 provided between the second sheet feeding rollers 25 and a transfer electrode described later.
In a copying operation, transfer sheet P in the selected sheet feeding cassette 23 is fed by the first sheet feeding roller 24, and guided by the guide plate 26 and conveyed to the second sheet feeding rollers 25. In this case, the second sheet feeding rollers 25 (referred to as registration rollers) are driven in timed relation in which the leading end of the toner image on the photoreceptor drum coincides with the leading end of transfer sheet P. In order to conduct an accurate copying operation, in accordance with the positional difference between the sheet feeding cassettes 23, transfer sheet P is temporarily stopped at the registration rollers 25 after it has been sent out from the sheet feeding cassette 23. Synchronously when an optical scanning operation is started, the registration rollers 25 are driven in timed relation in which the leading end of the toner image on the photoreceptor drum coincides with the leading end of transfer sheet P.
A transfer electrode 28 transfers the toner image on the photoreceptor drum 20 to transfer sheet P, and a separation electrode 29 separates transfer sheet P from the surface of the photoreceptor drum 20. Transfer sheet P separated by the separation electrode 29 is conveyed to a fixing unit 31 through a transfer sheet conveyance means 30, and fixed by a thermal fixing roller and a press roller. Then, transfer sheet P is discharged onto a sheet discharge tray 33 by a discharge roller 32.
After the image transfer operation has been completed, the photoreceptor drum 20 is cleaned by a cleaning unit 34 to remove the residual toner existing on the surface of the photoreceptor drum 20. The residual toner on the photoreceptor drum surface is scraped off by a blade 34a provided in the cleaning unit 34. In order to conduct the toner removal operation easily, a cleaning discharge electrode 35 is provided before the blade 34a. After the charging electrode 21, a charge erasing unit 36 to erase the charge with rays of light is provided being opposed to the photoreceptor drum 20. Numerals 37 and 38 are respectively a precharging exposure unit and a pretransfer exposure section.
Next, a transfer sheet feeding operation will be described in which transfer sheets are sent out from the sheet feeding cassette. FIG. 7 is a schematic illustration to explain a sheet feeding control operation conducted by the registration rollers. Like parts in each of FIGS. 6 and 7 are identified by the same reference character. As shown in FIG. 40, in the sheet feeding cassette 23, a plurality of trays 40 are disposed in order to accommodate various types of transfer papers.
When a copy button (not shown) is pressed, a transfer sheet P is sent out from a predetermined tray 40 in the sheet feeding cassette 23 by the first sheet feeding roller 24. The sent transfer sheet P is conveyed to the registration rollers 25 through buckling rollers 41. Since the registration rollers 25 are not rotated at this time, transfer sheet P is curved as shown in the drawing when the buckling rollers are rotated. When a large buckle of transfer sheet P is formed, the sheet feeding operation is stopped. In order to synchronize the sheet feeding operation with the optical system, it is necessary to stop transfer sheet P at the registration rollers 25 for a predetermined period of time.
The registration rollers 25 send out transfer sheet P in synchronization with the scanning operation of the optical system. Then, the conveyance of transfer sheet P is controlled so that the leading end of transfer sheet P can coincide with the leading end of the document image to be transferred.
In order to transfer the document image in a predetermined position on transfer sheet P after transfer sheet P has been sent out by the registration rollers 25 in synchronization with the scanning operation of the optical system, it is necessary that transfer sheet P is stopped at the registration rollers 25 (the registration section) for a predetermined period of time (referred to as a temporary stop time).
However, after the copying machine has been used for a long period of time, the first sheet feeding roller 24 and other conveyance units are worn out, so that slippage is caused when transfer sheet P is sent out to the registration roller 25 by the first sheet feeding roller 24. Therefore, transfer sheet P is delayed in arriving at the registration section, so that the temporary stop time can not be ensured. For the reason described above, there is a possibility that transfer sheet P does not arrive at the registration rollers 25 even after the scanning operation has been started by the optical scanning system, and transfer sheet P is jammed.